Experimental and numerical study of turbulent flow around a Fanwings profile

Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=tcfm20 Engineering Applications of Computational Fluid Mechanics ISSN: 1994-2060 (Print) 1997-003X (Online) Journal homepage: https://www.tandfonline.com/loi/tcfm20 Experimental and numerical study of turbulent flow around a Fanwings profile Slimane Benferhat, Tayeb Yahiaoui, Bachir Imine, Omar Ladjedel & Ondřej Šikula To cite this article: Slimane Benferhat, Tayeb Yahiaoui, Bachir Imine, Omar Ladjedel & Ondřej Šikula (2019) Experimental and numerical study of turbulent flow around a Fanwings profile, Engineering Applications of Computational Fluid Mechanics, 13:1, 698-712, DOI: 10.1080/19942060.2019.1639076 To link to this article: https://doi.org/10.1080/19942060.2019.1639076 © 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group Published online: 20 Jul 2019. Submit your article to this journal Article views: 33 View Crossmark data ENGINEERING APPLICATIONS OF COMPUTATIONAL FLUID MECHANICS 2019, VOL. 13, NO. 1, 698–712 https://doi.org/10.1080/19942060.2019.1639076 Experimental and numerical study of turbulent flow around a Fanwings profile Slimane Benferhata, Tayeb Yahiaouia, Bachir Iminea,OmarLadjedel band Ondřej Šikula b aLaboratory Aeronautics and Propulsive Systems, Département de Génie Mécanique, Faculté de Génie Mécanique, University of Science and Technology of Oran Mohamed Boudiaf, Oran, Algeria; bFaculty of Civil Engineering, Brno University of Technology, Brno, Czechia ABSTRACT The main objective of this paper is an experimental and numerical study of airflow on a propulsive wing also called ventilated wing or FANWING, which is a drone operating at low speed. To optimize the aerodynamic shape of the Fanwing, two different configurations of NACA4415 rectangular wing profile were realized. The first one is a wing where the Cross-Flow Fan is fitted directly to the leading edge with a classic niche. For the second one, we truncated the extension of the niche to create a profile without nose. Two flow velocities with constant fan rotation were used and observed in the range of −16° <α<+30°. A lift coefficient generated by the profiles increases and the drag coefficientdecreases,whilethedistributionofthepressurecoefficientontheuppersurfaceincreases abruptly because of the flow recirculation. The experiment was performed in a subsonic wind tunnel TE44 and numerical simulations in software Fluent 6.3.2.6. Both approaches are in good agreement. The visualization showed that the recirculation phenomenon occurs right after the discharge of the cross-flow fan. It reveals that the jet coming out of the fan causes a strong wake behind the profile and suppresses the boundary layer separation. ARTICLE HISTORY Received 21 March 2019 Accepted 29 June 2019 KEYWORDS Fanwing; aerodynamics; recirculation; eccentric vortex 1. Introduction Thephysicalstudyoftheflowaroundtheprofilesis of great interest in understanding its behavior in order to predict the phenomenon of dynamic stall and stability. These flows are complex and irregular generating swirling flows that can be detrimental in internal flows causingvibrationsandsystemdamage.Theyalsocanbe beneficialinexternalflowssuchascoolingorinaeronautics, producing lift in rotary wing aircrafts. The rotational flows are always captivating whether they are from a convective flow, generating a hurricane or formed by using a turbomachine or a rotor. One of these devices is the crossflow fan, where the air is sucked by the cross-flow fan and then stirred with the blades. The whirling air gives birth to a vortex inside the fan before being ejected radially. This vortex is eccentric. Its position and dynamics quantitatively affect the overall characteristics of the device (Element 2, Figure 1). The majority of the work carried out on Cross-Flow Fans is generally experimental, aimed at improving the aerodynamic performance. By modifying the geometric characteristics randomly, guided either by the stability of the system or by its size. Several research projects are the subject of intensive studies on its application in aeronautics. A detailed understanding of this flow field is essential. CONTACT Ondřej Šikula [email protected] For several years, the improved performance of rotorcrafts and rotary wing aircrafts capable of vertical takeoff and landing (VTOL) or a short takeoff and landing (STOL),hasledresearchtoinnovationandtheFanwing is one. He requires a short runway. The novelty of this type of propulsion is to move an aircraft using a cylindrical fan. Fanwing is a rare concept, working at low Reynolds numbers, developed and patented by Peebles (2001); it was inspired by the paddle steamers that sailed the Mississippi River. This type of aircraft with motorized lift uses a turbine engine mounted on the entire wingspan of a wing similar to a rigid wing. This is a different mode of operation from the conventional aircraft. This type of turbomachine is called cross-flow fan. It was patented by Mortier (1893). The rotation of the fan is used to accelerate the air flow on the profile. This action results in lift and propulsion while delaying the boundary layer separation, stemming from the acceleration of the incoming air and entering from the leading edge directed towards the trailing edge. The evolution of computer tools and digital methods has revolutionized the CFD. It has had an impact on fluid mechanics and aerodynamics flows. The development of this tool has enabled computational fluid dynamics to cover all domains related to fluid flows © 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ENGINEERING APPLICATIONS OF COMPUTATIONAL FLUID MECHANICS 699 Figure 1. Flow around Fanwing with à modified Naca 4415 profile. (internalandexternal).Fromarchitectureforbuilding construction (Mou, He, Zhao, & Chau, 2017)to hydrogen production (Akbarian et al., 2018)tointernal engine flows (Ardabili et al., 2018)toNano-fluid (Ramezanizadeh, Nazari, Ahmadi, & Chau, 2018), (Ghalandari, Koohshahi, Mohamadian, Shamshirband, & Chau, 2019). This revolution has led to the emergence of digitalaerodynamics,whichplaysaroleinthedesignand optimization of aircraft. 1.1. Principle of operation of the Fanwing The rotation of the blades (1), of the rotor (2) placed in the cavity (3) accelerates the air sucked in. It passes twice between the blades (4) (which transmits to the air its energy) where about 2/3 of the rotor diameter exceeds the upper surface of the profile and then directs the flow towards the rear of the upper surface (5). Which produce lift and propulsion simultaneously. They depend on the speed of rotation ωof the axis (6) while delaying the boundary layer separation resulting from the acceleration of the air. Depending on the shape of the leading edge (7) the air flow will be regulated and the lift and propulsion forces will be controlled. The trailing edge (8)forcestheflowtobeparalleltoitsplane(Coanda Effect) (Figure 1). This results in unusual aerodynamic parameters and specific qualities with a broad operating prospect. In collaboration with Peebles, Forshaw (1999)conducted the first basic Fanwing investigation at the Imperial College of London, where the experimental study was performed in the college’s wind tunnel. He showed that the lift increases by 50% compared to a conventional aircraft and does not stall even at high angles of attack. Kogler (2000) continued the experiment on the same prototype. He noticed that the Fanwing works well at low altitude, contrary to the observations of Forshaw. Kogler found that the wing was stalling for angles of attack above 20 degrees and suggested that in order to avoid the stall, it is necessary to have additional engine power. Modifying Peebles profile, Duddempudi, Yao, Edmondson, Yao, and Curley (2007)performsanew numerical simulation while reducing the thickness by 16 mm. She concludes that the results are comparable to those obtained by Kogler and predicts that the lift force is close with an error of about 6.55%, and the drag force is comparable to 12.59%. Both of these errors are generally acceptable. With the modified geometry, the refinement of the fineness increases by about 29.42%. She states that the stall occurs at higher angles of incidence and the position of the eccentric vortex inside the fan is the key element. Several studies on cross-flow fans were done. Toffolo (2005) observes an eccentric vortex formation inside the cross-flow fan, similar to that found in studies established on the same devices. The position of the eccentric vortex on the inside is the most important phenomenon to define these aerodynamic parameters. Tanaka and Murata (1994) noticed that the flow inside the fan is very complicated and difficult to predict the behavior. The numerical study makes it possible to evaluate these aerodynamic performances, as well as the flow details in particular in the center with more accuracy. However, Mazur (1984) concludes that the geometry of the cavity of the fan is more important than the parameters of the fan (Moon, Cho, & Nam, 2003). describe the Fanwing as a flying lawn mower. The similarity is the use of a tangential fan mounted along the leading edge ofthewing.Theystatethatinadditiontotheincrease of the coefficient of lift CL, the fan generates a thrust force that accelerates the flow of air towards the trailing edge. This phenomenon delays the boundary layer separation and the stall occurs at an incidence greater than 30°. Kentfield (2005) believes that, to obtain stability at high altitudes it would be necessary to add to the Fanwing drifts (equip the Fanwing with winglets on the wing and the tail of the aircraft) in order to avoid great moments of roll and the boundary layer separation which causes wingtip vortices. This addition allowed the Fanwing to hover and expand flight speeds and reduced its sound. To have the ability to fly slowly and safely in urban areas, it would only need a short runway for takeoff or landing. Its low sonority at low altitude allows effective monitoring at the times of peace, war or emergency situations (Douglas &Geoff,2009). When Ahad and Graham (2007) conducted flight tests of the Fanwing model, working on the model of Forshaw (1999) they determined that even if the aircraft did not fly under the conditions of a real plane, the tests are conclusive. The takeoff and landing distances are shorter, compared to conventional aircraft. By adding drifts on the wings to reduce the pitching moment they almost doubled their cruising speed and the model has more of 700 S. BENFERHAT ET AL. astablebehavior.Theyalsoestimateacriticalthreshold of use between −20° <α<+20°. Saracoglu and Paniagua (2015) concluded that the static pressure inside the cavity is relatively low, comparedtotheuppersurfacecausedbythepresenceofthe eccentric vortex inside the fan. When increasing the rotational speed increases the lift and thrust proportionally to themassflowofairforcedthroughthefanonthewing surface. AccordingtoAskariandShojaeefard(2015), the improvement of the coefficient of pitch moment shows that the aircraft is more stable at higher rotational speeds. In his investigation, Seyfang (2011)statesthatthe development of Fanwing has considerably increased the cruising speed. This investigation focused on the comparison of four different rotorcrafts. It showed that the Fanwing requires a very short take-off runway and offers an interesting performance, similar to that of helicopters and inclined rotor aircrafts. Kummer and Dang (2006) carried out another design by wake ingestion. Leroy and Smith (1993) demonstrated that,ifafanisplacedbehindamovingbody,itabsorbs the wake by re-energizing its flow. As a result, Kummer and Dang (2006) developed it, by working on a Gottingen 570 profile, they eliminated the fan support structure by enclosing the fan and installing it at the trailing edge. The fan recovers the energy of the flow by sucking its wake, allowingittoreducethepowerofpropulsionbysaving the energy of the engine. In 2010,Kummer,theSyracuseprojectandNASA, built a prototype of the Fanwing. Using CFD to solve Navier-Stokes 3D equations, a new polyhedral honeycomb mesh was invented to optimize fan housing and computation time. 2. Description of the solution 2.1. Experimental apparatus The experimental tests were carried out in the TE 44 closed circuit horizontal atmospheric wind tunnel at USTO Oran (Figure 2(a)). The test section of the wind tunnel is square 0.46 m ×0.46, and 1.2 m long. It is of a classic design where the speed can reach 60 m/s. The intensity of the turbulence is less than 1% (Technical document). Designed to measure the aerodynamic forces exerted ontheprofile,thewindtunnelisequippedwithaTE81 balance, connected by cables to strain gauges for the measurement of the lift, drag and pitch moment (Figure 2(d)). ThesewillbereadusingtheDATASLIMsoftwareonthe visualization interface shown in Figure 2(e) (the application software displays the pressure tapping points and the forces detected by the TE81 scale). For the pressure measurement, the machine is equipped with a TE44 DPS type sensor, which allows the recording of 20 static pressure taps of the flow. The experimental models in the test sections are shown in Figure 2(b,c). Figure 2. Experimental tests of Fanwing in USTO Oran. (a) Subsonic Wind tunnel TE44. (b) Free test section. (c) Closed test section. (d) Wind tunnel balance TE 81 and free test section. (e) Forces & pressure measurement. ENGINEERING APPLICATIONS OF COMPUTATIONAL FLUID MECHANICS 701 Figure 3. Fanwing study with à modified NACA 4415 profile. (a) Fanwing with niche. (b) Fanwing without niche. 2.2. Realization and description of the Fanwing models The profile selection for low and high Reynolds numbers and for low and high altitudes led us to choose an asymmetric profile, (the NACA 4415). The latter is used for wide-body aircraft. Abbott and Von Doehoff (1959)performed a study on the aerodynamic performance of Naca 4415. He stated that the stall angle is more than 16° and that the drag is positive. For our Fanwing study, the NACA 4415 profile is modified. The details shown in Figure 3(a,b) are drawn on SolidWorks, and the wings are constructed with Balsa aeromodelism wood. The aluminum fan consists of 22 curved blades. The chord and the thickness of the blades are 10 and 0.4 mm respectively. The outer diameter of the fan is 60 mm (Figure 4, Element 6). For our design, conventional profiles such as the Naca 4415 limits the size of the fan for the support of the fastening system and for high-speed applications because of the compressibility effects. The cross-flow fan is likely to create a pressure difference allowing the flow of air between upstream and downstream. The cross flow-fan supplies to the air a large part of the mechanical energy that it receives through the electric motor shaft. Figure 4shows the different components of the test bench. To make the experimental tests more reliable andrealistic.WeusedaPROTRONIKmaterialintended Figure 4. Components of the test bench. for aeromodelism. The system consists of a three-phase 4000 rpm electric motor (4), directly coupled to a fan (6) of 440 mm in span and 60 mm in diameter. A six-position controller (3) and a 70-amp speed controller (2) are used to adjust the rotational speed and the desired power with an electrical circuit using a 12-volt power supply (1). The system is fixed on two supports (7) acting as fixing and endplate so that the flow is two-dimensional (2D). These experiments were carried out on two models of wings of infinite span L=460 mm and two chords (c=160 mm and c=140mm)fortheprofilewithnicheandprofile without niche respectively. The aerodynamic parameters were measured at a rotational speed of n=3000 rpm and two Reynolds numbers Re =76,765 and 57,574; hence the two-speed rates that define the ratio of the fan rotational speed and the airflow speed (=1.55 and =1.18). According to Kogler (2000), this is the key parameter of the Fanwing 702 S. BENFERHAT ET AL. similitude ratio , i.e. two free stream velocity 7.98 and 6.07 m/s were used for the experimental study. This work is done at a range of incidence of −16° <∝<+30°. The airflow on the profiles are the turbulent and the viscosity this is assumed unchanged throughout the study. Re =U∞·c ν(1) TSR ==ωr U∞ (2) 2.3. Results and discussions of the experiments 2.3.1. Impacts of the fan rotational speed Given the low efficiency at high speeds, the choice of tangential fan is one of the major concerns. Published work on fan performance is generally 40% for simple geometries and 60% for more complex ones (Mazur, 1984). However, in Harloff’s (1979)research,anefficacyof 70–80% was studied. The equipment available in the laboratory did not allow us to vary the rotation. However, in our investigation, Kogler (2000)andAskariandShojaeefard (2015) claim that the coefficient of lift increases and the drag decreases rapidly. They are strongly influenced by the variation of the fan rotational speed. Saracoglu and Paniagua (2015) used a wide range of Tip Speed Ratio () and observed the effect of angular velocity. A significant increase in the lift and thrust coefficient was noted as a function of the increase in rotational speed. The lift coefficient reaches values up to 83 for very high rotational speeds, corresponding to peak speed ratios of approximately =30. Several studies on aerodynamic performance using a Navier-Stokes solver have been conducted. In the research of Chen and Lian (2015), a numerical investigation of vortex dynamics in an H-rotor vertical axis wind turbine is done, they note that the dimensionless number tip speed ratio () defined the range of wind turbines and it can be seen that as the tip speed ratio ()decreases more portion of the rotation cycle exceeds the static stall angle. 2.3.2. Influence of the Reynolds number To define the aerodynamic parameters of a wing, we drewthecurvesoftheliftanddragcoefficientsofthe wing. Then, we observed the pressure coefficient on the profile. (a) Lift coefficient The values of CL,areinjectedontheCLgraphs to estimate the relative error. Note that these errors are small for positive impacts and increases for large values of negative angles. In Figure 5, curves (a) and (b) represent the lift coefficientforthetwoReynoldsnumbers.Wenotethatit decreasesastheReynoldsnumberincreases.However, itisimportanttonotethatdespitethefactthatthelift coefficient is decreasing, the lift force increases when the Reynolds number is increased. This decrease of the lift Figure 5. Variation of the lift coefficients of the two Fanwing profiles with the bars of the relative error. ENGINEERING APPLICATIONS OF COMPUTATIONAL FLUID MECHANICS 703 Figure 6. Variation of the drag coefficients of the two Fanwing profiles with the bars of the relative error. coefficient results from the definition of lift, which is proportionaltothesquareofflowvelocity.Atanincidenceof 0°, the profile develops a high coefficient of lift; this is the characteristic of Fanwings. The CLincreases linearly with the angle of attack CL=CL(α), for the Fanwing without a niche. It reaches a maximum between the incidence of 8° and 14°, then suddenly drops; the flow no longer follows the shape of the profile. CL=2L ρ·S·U2(3) (b) Drag coefficient The error values of the drag coefficients CD,are introduced on the CDgraphs. They are relatively low, as predicted for lift. It remains low for both profiles. It increases for negative angles (from α>−8). The drag coefficient graphs (a) and (b) in Figure 6 show that the profile drag is completely negative for the fanwing without niche. For ∝=0° the value of CD=−0.57756. It acts as a propulsive force and it is the rotation of the fan that reduces it to become negative. While for fanwing with niche the CDvalues are low. For ∝=0° the value of CD=−0.15773. And can be used between −4° ≤∝≤+14°. The drag goes through a minimum where the propulsiveforceismaximum,andthenitvariesuntilitbecomes maximum where the propulsive force becomes minimal. For high incidences, the motor weakens and requires an increase in power to produce the necessary propulsive force. By taking more incidence (positive angle) the fan oftheprofilewithoutnichedropsspeed.Itencountersair resistance and the speed of rotation of the cross-flow fan decreases. To keep it constant at 3000 rpm increases the electrical energy of the motor. CD=2D ρ·S·U2(4) The curves of the lift coefficient in Figure 7(a) representtheprofilewithoutniche,whichismoreadvantageous than that with niche. They show that, even with negative incidence, the Fanwing is stable and produces a lift at −10°. Then the lift coefficient increases almost linearly with the angle of attack and reaches a maximum of +20°whereitdrops.Theflownolongerfollowstheshape of the profile; it is the phenomenon of stalling. For high Reynolds numbers, the lift coefficient can be smaller whereas the lift force is higher, and for low Reynolds numbers and longer monitoring and interesting, and for higher speeds the consumption of energy will be more significant. It follows from the curves of the drag coefficient of Figure 7(b) that the drag force generated by the profile increases with the Reynolds number and decreases with the rotation of the fan, until it becomes propulsion. This brings us to the goal we are seeking a low Reynolds surveillance aircraft. They show that the profile with niche has more resistance to air than that without a niche, therefore produces more drag than propulsion. This is attributed to the geometry of the fan cavity as claims Mazur (1984), it is the most important parameter. The drag coefficient decreases with the Reynolds number, while the drag force itself increases in the same way as the lift. 704 S. BENFERHAT ET AL. Figure 7. Comparison of the variation of lift and drag coefficients between the two Fanwing profiles. Figure 8. Positioning of the profiles in the closed test section for pressure coefficient measurement. (c) Pressure coefficient As shown in Figure 8, the location of the profiles in the closed test section for the measurement of the pressure coefficient. The shapes shown on the curves of Figures 9 and 10 illustrate the comparison of the distribution of the pressure coefficient at different incidences. Much of the lift occurs in the cavity by the effect of the rotation that is defined by the Magnus effect. We can see the evolution of the pressure gradient as well as the pressure jump observed on the upper surface of the Fanwing without niche, at the very beginning of the discharge of ENGINEERING APPLICATIONS OF COMPUTATIONAL FLUID MECHANICS 705 Figure 9. Distribution of the pressure coefficient of the Fanwing with niche. Figure 10. Distribution of the pressure coefficient of the Fanwing without niche. the fan. CP=p−p∞ (1/2)ρU2 ∞ (5) We deduce from this series of curves (Figure 11)that a stagnation point appears with a sudden jump in pressure on the upper surface of the profiles. Overall, even if they are similar in shape, the Fanwing with niche does not develop an important pressure gradient. Unlike the Fanwing without niche for which it is considerable along the entire profile and which is reflected on the lift. This jump is due to the geometry of the cavity and the sharp edge on the profile. To reduce this jump, Askari and Shojaeefard (2015) claims that the sharp edge is replaced by a smooth rounded edge. If the depression gap becomes important, there will be a risk of developing a critical speed that will give rise to a deformation of the wing. It is proposed to install a perforated deflector with variable opening to control the airflow. We can see that the distribution of the pressure coefficient depends on the geometry of the housing and thattheprofilewithoutnichehaslargenegativepressure values and develops large lift coefficients. 2.4. Observation of the flow by visualization The visualization was performed on the wing without niche at an incidence of zero (0°) and five (5°) degrees and at a =1.55 and Re =57,574, which aims to observe the flow and locate the eccentric vortex inside the Cross FlowFan,aswellasthewakeattheexitofthefan (Figure 12(a)). During the flow, a jet of smoke is initiated towards the fan. 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